1. Field of the Invention
The present invention relates to bending of a lead of an electric part or the like. More particularly, the present invention relates to a method of Z-bending an end portion of a lead of an electric part such as an IC, LSI, etc. and also pertains to a bender which may be effectively employed to carry out the lead bending method.
2. Description of the Prior Art
The mainstream of LSI mounting methods has been shifting from the conventional method employing DIPs (Dual In-line Packages) to the so-called surface mounting method in which a leaded electric part or device package is electrically connected to a conductor provided on the surface of a printed wiring board without using throughholes for insertion of lead wires. This shift is made to cope with an increase in the number of gates or the like as a result of the improvements of LSI chips, i.e., the achievement of high integration density, the increase in the number of functions thereof, and the achievement of highspeed operation. Examples of known surface-mounted packages include resin-sealed SOJ (Small Outline "J") leaded packages and PLCCs (Plastic Leaded Chip Carriers). Leads that are used in these packages are bent in a J or L shape and therefore capable of being mounted directly on a printed wiring board (PWB).
In general, bending of lead wires of ICs is effected by press working. Since lead wires are extremely thin, they are fragile and it is therefore necessary to use extreme care in bending them. The conventional bending method, which has heretofore been carried out by the present inventor, will be explained below specifically with reference to FIGS. 8 and 9. An LSI package 60 is fed to a fixed die 80 by a robot hand or work feeder (not shown) and placed in a cavity 81 provided in the surface of the die 80. At this time, the root portion 64 of a lead 65 of the LSI package 60 is placed on a press projection 82 that is provided at the outer periphery of the cavity 81. Next, a press machine (not shown) is driven to lower a punch 70 so that the lead 65 is pressed from the root portion 64, thereby bending it into a substantially Z-like shape, as shown in FIG. 9. Thus, the lead 65 is pressed into a substantially Z-like shape by being clamped between the side and upper surfaces 83 and 84 of the press projection 82 of the die 80 and the side and lower surfaces 71 and 72 of the punch 70. In this way, so-called Z bending is performed.
Incidentally, the lead 65 is plated with solder or tin in order to improve the electric conductivity required when it is mounted on a printed wiring board (PWB) and to enhance the adhesion of solder used to connect the lead 65 to the PWB. In the conventional Z bending method, the surface of the lead 65 is in contact with a part of the surface of the punch 70 and portions b and d of the lead 65 are ironed by the punch 70, that is, subjected to burnishing. In consequence, the portions a and c are dented, while the portions b and d are scratched due to the frictional contact. In an extreme case, the plating on the lead 65 is peeled off. Incidentally, in recent SO- and QFP-type LSI packages, leads are extremely thin, narrow and short, that is, the lead pitch is in the range of from 0.3 mm to 0.8 mm, and the lead length is in the range of from 1 mm to 3 mm. When such an LSI package is mounted on a printed wiring board, if any of a large number of leads has the plating peeled therefrom due to a dent or scratch, the lead cannot be electrically connected to the printed wiring board, resulting in the whole product being defective.
The peeling of the plating from a lead gives rise to another problem. That is, the punch and die of the bender are fouled with the tin or other soft metal peeled from the lead. The tin or other soft metal thus attached to the bending members acts as a resistance that causes the lead 65 to be pulled in the directions of the arrows e and f (see FIG. 9). As a result, a thin gold wire 62 bonded between an IC chip 61 and a lead frame 63 inside the LSI package 60 is cut off. In addition, the mold 66 of the package 60 may be cracked in the form of a microcrack g.
Accordingly, it has been conventional practice to remove the bender from the press machine and remove the metal peeled off the lead 65 and attached to the punch 70 and the die 80 by a manual operation. Thus, the prior art suffers from the problems that the lifetime of the bender is short and the cost is high.